1. Field of the Invention
This invention relates generally to apparatus for exciting a light source and more particularly to solid state driver circuitry for exciting a high intensity electrodeless light bulb.
2. Description of Related Art
There has been a long term need for improved light sources for both commercial and industrial use. The key parameters sought are efficiency, light quality, reliability and low cost. Various light sources have been developed in the past which address various aspects of these needs. To date, however, no light source has been found to be optimum in all respects, and therefore a huge commercial market awaits new techniques which offer significant improvements.
One such light source comprises an RF excited electrodeless light bulb which consists of a closed transparent glass sphere filled with a proprietary gas. Typically, the bulb contains an inert gas, such as argon, and an element from Group VI-A of the Periodic Table of elements, such as sulfur. When the gas is excited by an RF field, it glows with an intense white light. The efficiency of light output per unit input power in such a light bulb is considerably higher than other types of light sources and the quality of the light is unsurpassed for its similarity to bright sunlight. Because the bulb is hermetic with no electrodes, its cost is trivial and its reliability and useful life are exceptional.
Currently, a 1000 watt electrodeless bulb manufactured by Fusion Lighting, Inc. is driven at an RF frequency, for example 2.45 GHz, using magnetrons developed for microwave ovens. Magnetrons, while being relatively low in cost, have a much lower life expectancy, typically 4000 hours, than the bulb itself. While this operational life is acceptable for intermittent food preparation, it is not acceptable for lighting applications.
Another known method of exciting these types of electrodeless light bulbs is by means of an electrical coil which is wound around the outside of the light bulb so as to generate an RF excitation field. The coil is coupled to a driver which typically includes a source of RF energy and an RF power amplifier coupled to the excitation coil by means of a matching network.
With respect to the RF source for the excitation coil, it has recently become practical to generate hundreds of watts at S-Band using transistors. While a solid state power source using commonly available transistors can satisfy the reliability requirements of such a light driver, it exhibits inherent limitations with respect to overall efficiency, size, and cost. Presently available semiconductor technology, based on silicon or GaAs, also requires an expensive power supply to convert the AC line to low voltage at high current. Furthermore, such low voltage devices have limited ruggedness and require strict isolation from power line variations or static transients. Also, extensive heat sinking is required to supply CW power in the 50 to 100 Watt range at, for example, 2.45 GHz, and extensive heat sinking is required to dissipate waste heat at reliable device temperatures. Silicon carbide devices, however, do not have such limitations.
In the above referenced related applications U.S. Ser. No. 08/708,447, now U.S. Pat. No. 5,705,830 and U.S. Ser. No. 08/877,847, and U.S. Pat. No. 5,612,547, entitled, "Silicon Carbide Static Induction Transistor And Method", which issued to R. C. Clarke et al on Mar. 18, 1997, there is disclosed the details of a static induction transistor fabricated in silicon carbide and which finds particular application in this invention.